Digital-audio systems are often found in inexpensive personal computers (PC's). Audio inputs are typically sampled at a rate of 11.025, 22.05, or 44.1 KHz, or at rates of 8, 16, 32, or 48 KHz. These audio samples are often stored and altered in a PC before being played back, often at a different rate.
It may be desired to play back an audio sample at a different rate, or to mix audio samples having different sample rates. One of the audio samples must be converted to the sample rate of the other audio sample for mixing or playback to occur. Sample-rate conversion software or hardware is used to convert the sample rate.
While many approaches to sample-rate conversion are used, satisfactory results are not always achieved. The human ear is quite sensitive to slight distortions or discontinuities of a sound. Coarse sample-rate conversion produces noticeable distortion.
A particular problem occurs when audio streams are synchronized to independent free-running clocks. The clocks for the two audio streams may be generated from two different crystal oscillators. Even when the frequencies are the same, slight differences can occur between the two crystals. The frequency difference may be up to 1000 parts-per-million (ppm). For a nominal 11,025 Hz sample rate, the frequency can be as high as 11025+11.025 or 11036 Hz. When a 11036 Hz audio signal synchronized to one crystal oscillator is converted to a 11025 Hz rate, audio samples may be deleted after every thousand or so samples. Deleting audio samples can cause audible clicks or pops.